1. Field of the Invention
This invention relates to a dual homodyne detection system for measuring asymmetric frequency spectrum of an incident wave with respect to a carrier frequency thereof, and more particularly to a system of simple construction including two homodyne detectors and angle mirrors which system measures asymmetric frequency spectrum of an incident wave of optical or quasi-optical frequency range with a high reliability and a high signal-to-noise ratio.
2. Description of the Prior Art
The inventor disclosed a device for measuring asymmetric frequency spectrum in his Japanese Patent Laying-open Publication No. 55,266/1980 entitled "A Method for Detecting the Frequency Spectrum of a Received Wave Having an Asymmetric Frequency Spectrum", which method might be called a new homodyne method. Signals with asymmetric spectrum are encountered, for instance, in the measurement of nuclear fusion plasmas.
In general, to measure the ionic temperature or the electron density fluctuation of a nuclear fusion plasma, the plasma is irradiated with coherent millimeter wave or sub-millimeter wave so as to scatter the coherent wave by the plasma, and the desired measurement is taken by detecting the frequency spectrum of the scattered wave. The frequency spectrum of such scattered wave generally has an asymmetric distribution of upper and lower sideband-components relative to the frequency .omega..sub.i of the incident coherent wave.
The above-mentioned new homodyne method was developed in order to facilitate the measurement of such asymmetric frequency spectrum. FIG. 1A shows a device of the prior art for measuring the asymmetric frequency spectrum of the scattered millimeter wave based on the new homodyne method. In the figure, a part of the incident wave from a millimeter wave oscillator OSC is applied to a magic-T M.sub.i through a directional coupler D.sub.i, and the incident light is branched to two paths (a) and (b). The two paths (a) and (b) are completely symmetrical to each other relative to the magic-T M.sub.i except that the path (b) has a 90.degree. phase shifter PS inserted therein. Directional couplers D.sub.1 and D.sub.2 are connected to the paths (a) and (b) respectively. On the other hand, the coherent millimeter wave from the oscillator OSC is also applied to the transmitter horn PH.sub.i through the directional coupler D.sub.i, so as to irradiate a plasma PL.
In response to the irradiation, scattered wave E.sub.s (t) emanates from the plasma PL and is received by a receiver horn PH.sub.s. The scattered wave E.sub.s (t) is applied to a magic-T M.sub.r and branched toward the two directional couplers D.sub.1 and D.sub.2. At the directional couplers D.sub.1 and D.sub.2, the branched scattered waves E.sub.s (t) are coupled with the local coherent millimeter waves from the paths (a) and (b) respectively, and then applied to diode mixers X.sub.1 and X.sub.2. Thereby, homodyne detection is effected, and output intermediate frequency signals V.sub.1 (t) and V.sub.2 (t) are produced.
The scattered wave E.sub.s (t) has an asymmetric frequency spectrum with respect to the frequency .omega..sub.i of the coherent millimeter wave, as shown in the following equations (1) and (2). ##EQU1## Here, .vertline.E.sub.s.+-. (.omega.).vertline. represents amplitudes of the upper sideband (+) and lower sideband (-) higher harmonic components to be determined by the Fourier series, while .phi..sub..+-. represents phase angles of the upper sideband (+) and lower sideband (-) higher harmonic components.
The local coherent millimeter waves to be applied to the diode mixers X.sub.1 and X.sub.2 can be given by the following equations. EQU E.sub.l1 (t)=E.sub.l .multidot.cos (.omega..sub.i .multidot.t) (3a) EQU E.sub.l2 (t)=E.sub.l .multidot.sin (.omega..sub.i .multidot.t) (3b)
If it is assumed here that E.sub.l &gt;&gt;.vertline.E.sub.s.+-. (.omega.).vertline., the intermediate-frequency output signals V.sub.1 (t) and V.sub.2 (t) from the mixers X.sub.1 and X.sub.2 are given by the following equations (4a) and (4b). ##EQU2##
Here, .vertline.N.sub..+-. (.omega.).vertline. is a quantity proportional to the product E.sub.l .multidot..vertline.E.sub.s.+-. (.omega.).vertline., and can be determined from the record of the measured values of V.sub.1 (t) and V.sub.2 (t) over a time period T in the following manner.
At first, the auto- and cross-power-spectral densities G.sub.ik (.omega.) of the intermediate-frequency output signals V.sub.1 (t) and V.sub.2 (t) are determined by the following equation. ##EQU3##
The power spectrum densities of the upper and lower sideband components S.sub..+-. (.omega.) can be determined from the above auto- and cross-power-spectral densities G.sub.ik (.omega.) by using the following equation. ##EQU4##
FIG. 1B shows the formation of a device for measuring asymmetric frequency spectrum in the sub-millimeter wave range, i.e., the quasi-optical frequency range, based on the above new homodyne method for the measurement of the asymmetric frequency spectrum of the scattered wave in the millimeter wave range as explained above by referring to FIG. 1A. The formation of FIG. 1B is similar to that of FIG. 1A except that a far infrared radiation laser FIR for the quasi-optical frequency range is used instead of the millimeter wave oscillator OSC of FIG. 1A for the millimeter wave range, that beam splitters BS.sub.i are used instead of the directional couplers D.sub.i and the magic-T M.sub.i and M.sub.r of FIG. 1A, and that an optical phase shifter PS' consisting of angle mirrors with L-shaped reflective surfaces for elongating the optical path is used instead of the phase shifter PS.
The above-mentioned new homodyne method for measuring the asymmetric frequency spectrum has shortcomings in that its optical system is complicated due to the need of directional couplers and magic-T's or a comparatively large number of beam splitters BS, and that a high signal-to-noise ratio is hard to obtain because of the considerable loss in the scattered wave signal from the plasma PL at the beam splitters BS, especially at the beam splitters BS1 and BS2 for coupling the scattered light signal from the plasma PL with the local coherent light beam. Accordingly, there is a need for improvement of such new homodyne method.
To reduce the loss at the beam splitters BS1 and BS2, diplexers may be used. However, substitution of the beam splitters with the diplexers results in a more complexed optical system due to the increased number of different kinds of optical elements, so that such substitution is not desirable.